Advances in plasma processing have facilitated growth in the semiconductor industry. During plasma processing, diagnostic tools may be employed to ensure high yield of devices being processed. Radio frequency (RF) electrical measurements may be utilized as a diagnostic tool for monitor and/or control of plasma electrical properties to maintain tight control of process parameters during plasma processing.
During plasma processing, RF electrical measurements, e.g. voltage (V) and/or current (I), may be collected by a probe, e.g., TCP Kiyo V™ or TCP Kiyo 45 VI™ probe available from Lam Research Corp. of Fremont, Calif., for plasma diagnostic. The plasma diagnostic data from the probe may allow for determination of plasma potential, floating potential, electron density, and/or electron energy distribution function. However, accurate values for the plasma parameters may be difficult to determine because of the complexities involved in calibration and/or control of high RF voltage and/or current probe(s).
In order to calibrate a probe to measure high RF voltage and/or current, a high RF voltage and/or current test system with a high RF power generator is needed. A typical commercially available high power RF generator may deliver up to 500 volts at an accuracy of about 10 percent in a 50 Ohms system. However, the RF voltages being measured during plasma processing may exceed 6,000 volts peak with a minimum accuracy requirement of about 1.5 percent traceable back to a National Institute of Standards and Technology (NIST) standard. Thus, commercially available high power RF generators may not have the high RF power or the accuracy requirements for data collection employed by a probe for plasma diagnostic.
Referring to FIG. 1, a simplified schematic of a prior art RF delivery path 100 for the voltage test arrangement is shown. The RF power is supplied by a single air cooled 300 Watt generator 102, i.e. maximum output at 50 Ohms impedance, operating at about 13.56 MHz. The RF power output from generator 102 is routed by coaxial cables to a coaxial switch network 122.
As shown in FIG. 1, coaxial switch network 122 may be configured with a first switch (SW1) 104 and a second switch (SW2) 106. A 20 decibel (db) coaxial attenuator 110 is placed in the RF delivery path to enhance low power functionality by controlling SW1 104 and SW2 106. Attenuator 110 is employed to reduce power output from high RF power generator 102 to provide stability in the lower voltage test range.
For example, in the lower voltage range of about 200 to about 1,000 volts peak, SW1 104 and SW2 106 may be switched to select attenuator 110. For the higher voltage range of about 2,000 to beyond 6,000 volts peak, SW1 104 and SW2 106 may be switched to the high RF delivery path 108. In either case whether the attenuator is switched in or switched out, the power is routed to a V-load network 112.
In the example of FIG. 1, a position indicator 118 is coupled to SW1 104 and SW2 106. Position indicator 118 serves to monitor whether attenuator 110 has been selected to prevent hot switching. As the term is employed herein, hot switching refers to switching when there is output power coming out from the generator. Hot switching is not desirable during high RF power operations.
The signals coming from position indicator 118 are routed through control printed circuit board (PCB) 114. The signal conditions are read back over a data acquisition (DAQ) inputs/outputs (IO) 120 into a computer 116. Then the software algorithm in computer 116 interprets the signal conditions to determine whether to proceed or halt the test depending on whether the switches are selected correctly.
In general, commercially available high power RF generator 102 operates at about 50 Ohms with 300 watts of power. When operating a 50 Ohms system, enormous amounts of power, is needed to attain the desired high RF voltages, e.g. 10 kilowatts for 1000 volts peak to 360 kilowatts for 6,000 volts peak. In order for standard off-the-shelf RF generators to work, the RF generator may be integrated into a high impedance circuit to generate the higher voltages necessary for calibration of the probes. V-load network 112 is an example of a high impedance circuit that is tuned to deliver the required voltage range.
FIG. 2 shows a simplified schematic of a prior art voltage load network arrangement 200. In the example of FIG. 2, RF power is supplied by a 50 Ohms RF generator 202. The RF power signal is passed through a high impedance matched V-load network circuit 212 to generate high voltages necessary for plasma applications.
V-load network circuit 212 is configured with a first variable capacitor (C1) 204, a second variable capacitor (C2) 206, a third variable capacitor (C3) 208, and an inductor 210. The V-load network 212 is tuned to resonate at about 13.56 MHz. In this resonant system, the impedance needs to be matched between generator 202 and an output 216. Otherwise, generator 202 may run in an unstable condition and possibly shut down. With the input impedance from RF generator 202 of 50 Ohms, output 216 from V-load network circuit 212 is tuned to match the impedance of a probe 218 and a V-ref 214. The V-ref output signal 214 is sent to an RF voltmeter (RFVM) 215. Hence, in a matched network, the high impedance allows very high voltages to be sustained at output node 216. However, in order to calibrate probe 218, the high voltage output needs to have better accuracy than the capability of off-the-shelf measurements. For example, plasma applications require the measurement accuracy to be within about 1.5 percent over the range of high RF voltages being measured.
Referring to FIG. 3, a simplified schematic of a prior art RF voltage control arrangement 300 is shown. In the example of FIG. 3, a software algorithm in a computer 316 may send a command to a data acquisition board (DAQ) 320 to output an analog signal to drive a software-defined set point 324 to a high power RF generator 302. Hence, set point 324 may instruct generator 302 how much power to output.
The power signal output from generator 302 is routed through a switched network 322 to a V-load network 312. From V-load network 312, the signal, V-ref output 314, is measured. The V-ref-output signal 314 is sent to an RF voltmeter (RFVM) 315. The signal from RFVM 315 is sent as data to a general-purpose interface bus (GPIB) 318 and is read by computer 316.
In aforementioned closed-loop RF voltage control arrangement, data from GPIB 318 is compared with software-defined set point 324. For example, if a voltage at the V-load network 312 of 200 volts peak is desired, a software-defined set point 324 of 200 volts peak may be set. In a closed-loop control, for example, the voltage value from the data coming back over GPIB 318 may be compared to generate the control signal, DAQ 320, going back to RF generator 302. The process may be iterated through a control loop algorithm to achieve a V-ref output within the desired accuracy, e.g., 1.5 percent, of software-defined set point 324.
Unfortunately, the aforementioned prior arts suffer from a few deficiencies. In the case of commercially available RF generator, the voltages are in the ranges of up to about 500 volts peak. The 500 volts peak range is not high enough for the plasma applications. In addition, the voltage measurement accuracy of about 10 percent from commercially available RF generators may be inadequate. In the case where commercially available RF generator has been integrated into a V-load network, the voltage range and accuracy are within acceptable limits for plasma application. However, the high RF voltage test system does not have RF current measurement capabilities. Therefore, only voltage probe may be calibrated by the prior art high RF voltage test system.